Dynamic forward error correction in utra systems

ABSTRACT

A UE transmitter in a 3G UTRAN wireless communication system performs dynamic link adaptation (DLA) with dynamic semi-static parameters for overcoming RF propagation difficulties. Separate transport channels (DCH) are defined for each semi-static parameter, including forward error coding (FEC) coding type and rate. When data rate is decreased during DIA, a TFC is selected having the desired FEC coding type and rate. Since this adjustment occurs at each TTI, mapping of data packet codes in each timeslot on the physical channel includes the benefit of FEC rather than reduced data rate alone. This permits improved SIR in a timeslot that may be experiencing RF propagation difficulties during the UL mapping process.

CROSS REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims priority from U.S. ProvisionalApplication No. 60/397,360, filed Jul. 19, 2002, which is incorporatedby reference as if fully set forth.

BACKGROUND

[0002] The proposed invention relates to UMTS 3^(rd) Generation (3G)wireless communications. More specifically, it considers the TimeDivision Duplex (TDD) mode of operation using dynamic link adaptation(DLA).

[0003] A variety of services, such as video, voice and data, each havingdifferent Quality of Service (QoS) requirements, can be transmittedusing a single wireless connection. This is accomplished by multiplexingseveral transport channels onto a coded composite transport channel(CCTrCh). The CCTrCH is then mapped onto physical channels for transportover the air interface. Each transport channel is associated with atransport format set (TFS), which defines a set of allowed transportformats (TF). Parameters such as transport block size and transportblock set size are considered dynamic since they can vary within a TFS.In contrast, semi-static parameters cannot be dynamically changed for agiven transport channel. Rather, they can only be changed after RadioResource Control (RRC) signaling has been exchanged between the userequipment (UE) and the UMTS Terrestrial Radio Access Network (UTRAN).The time expenditure of this exchange to adjust semi-static parameterscan have unacceptable consequences with respect to timely mitigation ofan RF propagation failure.

[0004] Forward error correction (FEC) coding type and rate aresemi-static parameters that are identical for each TF within a TFS. AnFEC coding rate of ½ indicates roughly 2 times as many bits are requiredto transmit 1 bit of information, while a ⅓ rate means there are about 3times as many bits. A coding rate of ½ allows one extra FEC bit to beadded for each data bit. For coding rate ⅓, two extra FEC bits are addedfor each data bit. This allows the timeslot to tolerate a lower SIR.

[0005] There are a variety of possible combinations when multiplexingseveral transport channels onto a CCTrCh. A particular transport formatcombination (TFC) specifies the transport format of each of themultiplexed channels. A TFC set is a set of allowed TFCs.

[0006] A transport format combination indicator (TFCI) is an indicatorof a particular TFC, and is transmitted to the receiver to inform thereceiver which transport channels are active for the current frame. Thereceiver, based on the reception of the TFCIs, will be able to interpretwhich physical channels and which timeslots have been used. Accordingly,the TFCI is the vehicle which provides coordination between thetransmitter and the receiver such that the receiver knows which physicaltransport channels have been used.

[0007]FIG. 1A shows a UTRA protocol stack, which contains the followinglower layers: radio link control (RLC), medium access control (MAC) andphysical (PHY).

[0008] The RLC layer delivers logical channels bearing controlinformation to the MAC layer. These channels are the dynamic controlchannel (DCCH), which includes set-up information, and the dynamictraffic channel (DTCH), which carries user data such as voice and data.

[0009] The MAC layer maps the logical channels DCCH and DTCH todifferent transport channels (DCHs), which are then delivered to the PHYlayer. The MAC layer is responsible for selecting the TFC forcombination of transport channels DCH within the CCTrCH. This selectionoccurs at every transmission time interval (TTI), which is the period oftime for one data burst. For example, a 20 ms TTI represents atransmittal of data specified in the TF every 20 ms (typically amountingto two 10 ms frames). Typically, there are 15 timeslots in each frame.The TFC selection is based on the amount of buffered data of eachlogical channel and the UE transmission power on the uplink (UL)communication. The TFC defines all of the dynamic and semi-staticparameters for each transport channel within the CCTrCH. The selectedTFC and associated data for each UL CCTrCH is provided to the physicallayer for transmission. If the physical layer subsequently determinestransmission of this TFC exceeds the maximum or allowable UEtransmission power, a physical status indication primitive is generatedto the MAC to indicate that maximum power or allowable transmissionpower has been reached.

[0010]FIG. 1B shows a block diagram of the PHY layer combining transportchannels DCH_A, DCH_B and DCH_C on the CCTrCH and mapping them intophysical channels for transmission over the air interface. A data burstoccurs as one coded packet of data is mapped in one time slot on thephysical channel. The PHY layer is responsible for performing thechannel coding of transport channels DCH, including any forward errorcorrection (FEC). Among the parameters contained in the TFC are thedefined FEC coding types and rates. The system chooses, on a TTI basis,which transport channels will be active and how much data will betransmitted in each one. That is, the TFC selection is fixed for theduration of the TTI, and can only be changed at the commencement of thenext TTI period. The TFC selection process takes into account thephysical transmission difficulties, (maximum allowable power being one),and reduces the physical transmission requirements for some timeduration.

[0011] After the multiple transport channels are combined into a singleCCTrCh, the CCTrCh is then segmented and those segments are mappedseparately onto a number of physical channels. In TDD systems, thephysical channels may exist in one, or a plurality of differenttimeslots, and may utilize a plurality of different codes in eachtimeslot. Although there are as many as 16 possible codes in a timeslotin the downlink, it is more typical to have, for example, 8 codes in aparticular downlink in a particular timeslot. A connection can beassigned as many as 16 codes in a downlink timeslot. In the UL, the UEis limited to using two codes in any particular timeslot. There are anumber of physical channels defined by a plurality of codes in aplurality of timeslots. The number of physical channels assigned perconnection can vary.

[0012] In the UL, there are rarely more than two codes in a particulartimeslot. In any event, there are a number of physical channels definedby a plurality of codes in a plurality of timeslots. The number ofphysical channels can vary.

[0013] Dynamic link adaptation (DLA) is a fast adjustment mechanismperformed by the UE to combat difficult RF propagation conditions. Whena UE reaches its maximum transmission power, it can reduce its datarate, typically by ½, in an attempt to correct signal to interferenceratio (SIR), by restricting its TFC set to combinations having lowerpower requirements. For example, in a simple case having a singletransport channel, and the TFC corresponding to the allowed transportformats of the transport channel DCH, such a transport channel maysupport data rates of 0, 16, 32, 64, and 128 kbps. In this example theTFC set would be (TF0, TF1, TF2, TF3, TF4), where TF0=0 kbps, TF1=16kbps, TF2=32 kbps, TF3=64 kbps, TF4=128 kbps. Since transmitting at ahigher data rate requires more power, the data rate is limited duringtimes of congestion by restricting the TFC set to (TF0, TF1, TF2, TF3).This eliminates the possibility of the higher data rate TF4 being used.Blocked TFCs may be later restored to the set of available TFCs byunblocking them in subsequent periods when the UE transmission powermeasurements indicate the ability to support these TFCs with less thanor equal to the maximum or allowed UE transmission power.

[0014] In the 3GPP UTRAN TDD standard, it is specified that physicalresources (i.e., data) must be assigned in the PHY layer in sequentialorder, first by timeslot and then by code. Thus, during each data burst,the first code of the first timeslot is assigned, then the second codeof the first timeslot and so on until the first timeslot is completelyassigned. The assignment of data continues with the first code of thenext consecutive timeslot, the second code of that timeslot, and so onfor the necessary number of available timeslots and codes until dataresource requirements are satisfied. Upon degraded RF conditions, DLAdecreases the data rate and hence reduces the amount of requiredphysical resources per TTI. However, the UE assigns physical resourcesto timeslots within the frame in consecutive order, regardless of RFconditions for a particular timeslot. As a result, if the first fewtimeslots are the ones having poor SIR, the later timeslots withpotentially more favorable RF conditions are not utilized orunderutilized.

SUMMARY OF THE INVENTION

[0015] A UE transmitter in a 3G UTRAN wireless communication systemperforms dynamic link adaptation (DLA) with dynamic semi-staticparameters for overcoming RF propagation difficulties. Separatetransport channels (DCH) are defined for each semi-static parameter,including forward error coding (FEC) coding type and rate. When datarate is decreased during DLA, a TFC is selected having the desired FECcoding type and rate. Since this adjustment occurs at each TTI, mappingof data packet codes in each timeslot on the physical channel includesthe benefit of FEC rather than reduced data rate alone. This permitsimproved SIR in a timeslot that may be experiencing RF propagationdifficulties during the UL mapping process.

DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1A shows a representation of a UTRA protocol stack of layersand channels.

[0017]FIG. 1B shows a block diagram of transport channels being mappedin the physical layer.

[0018]FIG. 2 shows a flowchart for a dynamic FEC method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0019] Although the following description of the present invention iswithin the context of TDD, it is applicable to both FDD and TDD modes ofoperation. DLA enhanced by dynamic forward error correction (FEC) isuseful to either an FDD or TDD UE that reaches maximum transmissionpower.

[0020] The UE transmits both control plane information of the dedicatedcontrol channel (DCCH) and user plane data of the dedicated trafficchannel (DTCH) on the same connection. Table 1 shows a UE's TFC setsimplified for illustrative purposes, comprising five transport channelsDCH1, DCH2, DCH3, DCH4 and DCH5. For this example, the transportchannels are mapped by the MAC layer upon radio access bearerestablishment (i.e., UE call setup) such that the DCCH is mapped to DCH1and the DTCH is mapped to one from the group DCH2 to DCH5. The transportchannels DCH2 to DCH5 have user plane data that is predefined forsemi-static parameters by a system radio network controller (RNC). Thesetransport channels DCH2 to DCH5 can easily be stored by the RNC in alookup table.

[0021] As shown in Table 1, a TFCI value is assigned to each possibleTFC and the presence of control data for each channel is indicated by‘X’. In this example, DCH2 to DCH5 are mutually exclusive, and hence,never multiplexed together onto the CCTrCh. The CCTrCh, therefore, nevercontains more than one user plane DCH. TABLE 1 TFC Set with mutuallyexclusive DTCH mapping DCCH DTCH TFCI DCH1 DCH2 DCH3 DCH4 DCH5 1 X X 2 XX 3 X X 4 X X 5 X 6 X 7 X 8 X

[0022] In this example, the semi-static parameters assigned to transportchannels are forward error correction (FEC) coding type and ratecombinations. In a 3G UTRAN system, there are typically four FEC codingcombinations: no coding, convolutional ½ rate, convolutional ⅓ rate andturbo ⅓ rate. Accordingly, the transport channels in FIG. 2 are definedas DCH2=no coding; DCH3=convolutional ½; DCH4=convolutional ⅓; andDCH5=turbo ⅓.

[0023] The UE can dynamically change the TFC every TTI, depending on thedesired FEC coding. When a high coding rate is desired, such asconvolutional ⅓, the UE selects a TFC containing DCH4, by setting TFCI=2or 6. When a lower rate is desired, such as convolutional ½, the UEselects a TFC containing DCH3, by setting TFCI=3 or 7. All five channelsDCH1, DCH2, DCH3, DCH4 and DCH5 are defined, but only one of the userplane transport channels DCH2 to DCH5 will be mapped onto the CCTrCh,depending on the value of TFCI. The control plane transport channel DCH1is optionally mapped onto the CCTrCH.

[0024] When used in conjunction with DLA, the dynamic control of the FECcoding as described above maintains the same number of physicalresources for active timeslots while reducing their transmission powerrequirements. More specifically, the data rate is reduced by DLA when,due to poor SIR, it is decided that the current number of PHY channelscannot be supported. Although the rate is reduced in conventional DLA,there may not be an improvement in SIR if the timeslot experiencing highinterference is the first timeslot in which the user data istransmitted. Conventional DLA would continue reducing the rate until thenumber of bits transmitted in the first timeslot were reduced. With thelesser data rate, less timeslots and codes of timeslots are assigned,leading to under utilized PHY channel capacity. However, with dynamicadjustment of the FEC coding operating concurrently, those unassignedtimeslots and codes of timeslots are available to accept the additionalFEC bits. Thus, the data mapped on the PHY channels will have improvedSIR as a consequence of the adjusted FEC coding, in addition to thereduced data rate by the DLA. By allocating more FEC bits, the requiredtransmission power is reduced for the same target quality of service(QoS). Furthermore, the number of PHY channels can be maintained at fullcapacity, which takes advantage of all possible timeslots, so that thosehaving the best RF propagation potential are not eliminated fromcontention during mapping on the UL.

[0025] The present invention is not limited to dynamic control of asingle semi-static parameter. Alternative embodiments involving dynamiccontrol of any semi-static parameter are within the scope of the presentinvention. Examples of these parameters are the rate matching parameterand cyclic redundancy code (CRC) size. The UE must be configured suchthat a logical channel can be mapped to one of many transport channels.

[0026]FIG. 2 shows a flowchart for a dynamic FEC method. In step 201,the various semi-static parameters, such as FEC coding type and rate,are determined and defined for potential mapping as transport channelsDCH. These are stored in a lookup table in step 202 by the RNC. At step203, upon UNC setup, the RNC creates a set of TFCs such that eachsemi-static parameter is represented mutually exclusive for each TFCI.In step 204, the MAC of the UE selects the TFC from the TFC set havingthe optimum semi-static parameters for the present UE transmission powerconditions. At step 205, the logical channels DTCH and DCCH are mappedas transport channels DCH to the CCTrCh by multiplexing based on thedecision of step 204, and the appropriate TFCI is mapped onto the UE'stimeslot to indicate the mapped TFC for the UL communication. Steps 204and 205 repeat at every TTI on the UL, concurrently with DLA, todynamically adjust FEC or other semi-static parameters within theselected TFC.

What is claimed is:
 1. In a TDD UTRAN wireless communication systemhaving UE receivers communicating data transmissions via multiplexedtransport channels in combinations thereof on a CCTrCh, a method fordynamically varying the combinations of transport channels, comprising:configuring mutually exclusive dedicated transport channels based onsemi-static transport parameters; and mapping data to a channelselectively based on the preferred semi-static transport parameter. 2.The method of claim 1, whereby the configuring of transport channels isperformed by a radio network controller (RNC) further comprising:defining transport format combinations (TFCs) each with a mutuallyexclusive semi-static parameter; storing the TFCs in a lookup table. 3.The method of claim 2, whereby the UE medium access control (MAC) layerdetermines the appropriate semi-static parameter based on UEtransmission power, further comprising: dynamically selecting a TFC withthe desired semi-static parameter at every transmission time interval(TTI).
 4. The method of claim 1 wherein the semi-static transportparameter is forward error correction (FEC) coding type and rate.
 5. Themethod of claim 1 wherein the semi-static transport parameter is cyclicredundancy code (CRC) size.
 6. The method of claim 1 wherein thesemi-static transport parameter is rate matching.